This thing of darkness / Harry Thompson.

Robert FitzRoy is given the captaincy of HMS Beagle, surveying the wilds of Tierra del Fuego. He is aged twenty three, and suffers from hereditary manic depression. He takes on a passenger, a young trainee cleric and amateur geologist named Charles Darwin to follow his two ambitions one being that black and white men are equal, and that he can prove the truth of the Book of Genesis. This is the story of a deep friendship between two men, and the twin obsessions that tore it apart, leading one to triumph and the other to disaster.